This invention relates generally to photosensitive semi-conductor devices, and more particularly to photodiodes.
Photosensitive semi-conductors are used to translate optical information into electrical signals for processing. One type of photosensitive semi-conductor device is a photodiode which generates a current proportional to the light illuminating its photosensitive surface. Typically, amplifiers are required to increase the electrical current generated by a photodiode to a level sufficient for processing.
Photosensitive semi-conductors constructed of silicon vary in their sensitivity to different wavelengths of light. For example, a silicon-based photodiode typically generates more current from the same wattage of a longer wavelength of light than it would for the same wattage of a shorter wavelength of light. Therefore, the signal from a photodiode receiving a certain wattage of light of a shorter wavelength must be amplified more than the signal from an identical photodiode receiving the same wattage of light of a longer wavelength. In addition, optical filters placed directly on the photosensitive surface, with different transmission properties create a further disparity between wavelengths of light, and their resulting signal currents.
In many applications for photodiodes, an array of photodiodes is used with individual photodiodes in the array receiving specific wavelengths of light, or with individual or groups of photodiodes having filters for different wavelengths of light placed directed on the photodiodes. The problem then arises of having to provide different amounts of amplification for the signals from the individual photodiodes.
One application for photodiodes or arrays of photodiodes is in analyzers for blood and other bodily fluids. Certain characteristics of the fluid can be determined from the degree to which the fluid transmits, reflects, and absorbs different wavelengths of light. For example, in a blood analyzer, a light source is placed on one side of a fluid sample with an array of photodiodes on the opposite side of the sample. The photodiode array typically has a filter in front of it which directs different wavelengths of light to different photodiodes in the array. In other words, each individual photodiode only receives light of a discrete wavelength. The signals generated by each photodiode in the array are then amplified to a level sufficient for processing. Information about the sample of fluid can then be determined by analyzing the amount of each wavelength of light which was detected by the photodiode array.
One approach previously utilized in such applications is to have amplifiers of different ranges for each of the photodiodes. Again, that is necessary because the photodiodes which receive the shorter wavelengths of light require much greater amplification of their signals than do the photodiodes which receive the longer wavelength of lights. In addition, optical filters placed directly on the photosensitive surface, with different transmission properties create a further disparity between different wavelengths of light, and their resulting signal currents. However, it is costly to provide amplifiers with different amplification powers for each photodiode in the array.
Another possible solution to this situation is to "energy match" each photodiode in an array for a particular application so that the same wattage of a shorter wavelength of light applied to one photodiode creates the same amount of current (signal) as the same wattage of a longer wavelength of light applied to another photodiode. Each "energy matched" photodiode can then use an identical amplifier.
One way to energy match the photodiodes is to use a reticule (an opaque material with a slit in it) to limit the amount of light which is permitted to strike the photosensitive surface of each photodiode to varying degrees depending upon the wavelength of light which will be received by that particular photodiode. In the past opaque glass, metal or plastic reticules have been used for this purpose. However, these reticules have been relatively thick with the effect that if the light source is not perfectly perpendicular to the photosensitive surface the slits in the surface of the reticule block out some portion of the available light signal due to deflection. The possibility of deflection requires that the light source be extremely stable and exactly aligned. Metal reticules may also cause the additional problem of creating unwanted capacitance.
It is therefore desirable to have a way of regulating the amount of filtered or unfiltered light impinging on the array which does not block some portion of the available light if the light source is not perfectly perpendicular to the photosensitive surface and also does not cause any capacitance.